The present invention relates to a solid electrolytic capacitor and a method for manufacturing the same, and more particularly to a solid electrolytic capacitor using a conductive polymer as a solid electrolyte and a method for manufacturing the same.
In general, the conventional solid electrolytic capacitor is that a molding obtained by increasing a surface area of a film-forming valve metal such as tantalum or aluminum by, for example, sintering a fine powder thereof or etching a rolled foil thereof is used as one electrode (anode) in counter electrodes, and a valve metal oxide layer (Ta.sub.2 O.sub.5 or Al.sub.2 O.sub.3) formed on the anode by an electrochemical anodization or the like is used as a dielectric layer. A conductive layer is formed on the oxidized dielectric layer, and this is used as another electrode (cathode) in the counter electrodes. In this case, a solid electrolyte is interposed between the cathode conductor layer and the oxidized dielectric layer to form part of the cathode.
A metal oxide layer such as manganese dioxide or lead dioxide (PbO.sub.2) as disclosed in, for example, JP-A-3-163814 (the term "JP-A" as use herein means a Japanese Patent unexamined published application), or 7,7,8,8-tetracyanoquinodimethane complex salt (TCNQ complex salt) as disclosed in, for example, JP-A-58-79255 has conventionally been used as the solid electrolyte.
It is preferred for the solid electrolyte to have a function which electrically connects between the entire surface of the dielectric layer on the molding surface and the cathode conductor layer, and also a function which restores an electrical short circuit due to inner defect of the oxidized dielectric layer. For this reason, a metal which has a high conductivity but does not have a dielectric material repairing function cannot be used as the solid electrolyte, and manganese dioxide which transfers into an insulator by, for example, heat generated by short circuit electric current has been used. However, in the capacitor using a metal oxide such as manganese dioxide as a solid electrolyte, since the conductivity of the metal oxide is not sufficiently low, impedance at a high frequency region is large. Further, the capacitor using the TCNQ complex salt is poor in heat resistance due to that the TCNQ complex salt is liable to be thermally decomposed.
Recently, development on novel materials has been progressing in a field of polymer, and as a result, a conductive polymer exhibiting a conductivity is developed by doping a conjugated polymer such as a polypyrrole, a polythiophene or a polyaniline with an electron donative or electron attractive compound (dopant). In view of the fact that the polypyrrole shows a high conductivity and has a good stability with the passage of time, a solid electrolytic capacitor using the polypyrrole as the solid electrolyte material for cathode is developed as disclosed in, for example, JP-A-63-158829.
Further, JP-A-3-18009 proposes a method for manufacturing a solid electrolytic capacitor, wherein a conductive polymer layer by chemically oxidative polymerization of the polypyrrole is formed on the surface of an oxidized dielectric layer, and a graphite layer and a silver paste layer are successively formed thereon.
However, in the capacitor using the above-described conductive polymer as a solid electrolyte, the conductive polymer is formed in the form of a thin film, resulting in making the surface smooth. Therefore, there are problems that adhesiveness between the conductive polymer layer and the o graphite layer becomes poor, and an equivalent series resistance (ESR) in a high frequency region and a tangent of loss angle (tan .delta.) are large.